Ziqiu Chen

A combined Gigahertz and Terahertz (FTIR) spectroscopic investigation of meta-D-phenol: observation of tunnelling switching

ABSTRACT We report results on the dynamics of tunnelling switching based on a high-resolution spectroscopic investigation of meta-D-phenol in GHz and THz ranges. The pure rotational spectra were recorded in the range of 72–117 GHz and assigned to the localized syn- and anti-structures in the ground and the first excited torsional states. Specific torsional states were unambiguously assigned by comparison of the experimental rotational constants with theoretical results from quasiadiabatic channel reaction path Hamiltonian (RPH) calculations. The torsional fundamental νT at ≈ 309 cm−1 and the first hot band (2νT – νT) at ≈ 277 cm−1 were subsequently assigned in synchrotron based high-resolution Fourier transform infrared (FTIR, THz) spectra. The analyses provided accurate spectroscopic constants of all six states involved. It was found that the 2νT states are interacting through anharmonic resonances, indicating tunnelling switching as predicted by theory. Furthermore, tunnelling–rotation–vibration transitions were assigned and the tunnelling splitting in 2νT was determined as 1.72450(17) cm−1. This key result allowed the assignment of two Q branches at 275.21303(9) and 277.67127(9) cm−1 to vibration-tunnelling transitions of the (syn ← anti) type, hence confirming tunnelling switching dynamics in m-D-phenol. The ground-state energy difference of the syn- and anti-isotopomers is obtained experimentally as E0 (syn) - E0 (anti) = (hc)0.82 cm−1 in satisfactory agreement with the theoretical ab initio predictions of (hc)1.5 cm−1 given the small absolute values arising from the tiny zero point energy effects. The results are discussed in relation to further fundamental aspects of tunnelling in slightly asymmetric potentials including the effects of the parity violating electroweak interaction in chiral molecules.

A molecular quantum switch based on tunneling in meta-D-phenol C6H4DOH

We introduce the concept of a molecular quantum switch and demonstrate it with the example of meta-d-phenol, based on recent theoretical and high-resolution spectroscopic results for this molecule. We show that in the regime of tunneling switching with localized low-energy states and delocalized high-energy states the molecular quantum switch can be operated in two different ways: (i) a quasiclassical switching by coherent infrared radiation between the two isomeric structures syn- and anti-m-d-phenol; and (ii) a highly nonclassical switching making use of bistructural quantum superposition states of the syn and anti structures, which can be observed by their time-dependent spectra after preparation.

A COMBINED GIGAHERTZ AND TERAHERTZ SYNCHROTRON-BASED FOURIER TRANSFORM INFRARED SPECTROSCOPIC INVESTIGATION OF ORTHO-D-PHENOL

Tunneling switching is a fundamental phenomenon of interest in molecular quantum dynamics including also chiral molecules and parity violation.a,b,c Deuterated phenols have been identified as prototypical achrial candidates.d We report the high resolution spectroscopic investigation of the ortho-D-phenol in the GHz and THz ranges following our recent discovery of tunneling switching in its isotopomer meta-D-phenol.e Here we report new results on ortho-D-phenol.The pure rotational spectra were recorded in the range of 72-117 GHz and assigned to the synand antistructures in the ground and the first excited torsional states. Specific torsional states were assigned based on a comparison of experimental rotational constants with the quasiadiabatic channel reaction path Hamiltonian (RPH) calculations. The torsional fundamental at 308 cm−1 and the first hot band at 275 cm−1 were subsequently assigned. The analyses of pure rotational and rovibrational spectra shall be discussed in detail in relation to possible tunneling switching.

HIGH RESOLUTION GHZ AND THZ (FTIR) SPECTROSCOPY AND THEORY OF PARITY VIOLATION AND TUNNELING FOR 1,2-DITHIINE (C4H4S2) AS A CANDIDATE FOR MEASURING THE PARITY VIOLATING ENERGY DIFFERENCE BETWEEN ENANTIOMERS OF CHIRAL MOLECULES

We report high resolution spectroscopic results of 1,2-dithiine-(1,2-dithia-3,5-cyclohexadiene, C4H4S2) in the gigahertz and terahertz spectroscopic ranges and exploratory theoretical calculations of parity violation and tunneling processes in view of a possible experimental determination of the parity violating energy difference DpvE in this chiral molecule. Theory predicts that the parity violating energy difference between the enantiomers in their ground state (DpvE C 1.1 10 (hc) cm ) is in principle measurable as it is much larger than the calculated tunneling splitting for the symmetrical potential DE o 10 24 (hc) cm . With a planar transition state for stereomutation at about 2500 cm 1 tunneling splitting becomes appreciable above 2300 cm . This makes levels of well-defined parity accessible to parity selection by the available powerful infrared lasers and thus useful for one of the existing experimental approaches towards molecular parity violation. The new GHz spectroscopy leads to greatly improved ground state rotational parameters for 1,2-dithiine. These are used as starting points for the first successful analyses of high resolution interferometric Fourier transform infrared (FTIR, THz) spectra of the fundamentals n17 (1308.873 cm 1 or 39.23903 THz), n22 (623.094 cm 1 or 18.67989 THz) and n3 (1544.900 cm 1 or 46.314937 THz) for which highly accurate spectroscopic parameters are reported. The results are discussed in relation to current efforts to measure DpvE.